1. Field of Invention
The present invention relates to a nonvolatile memory device, and more particularly, to a phase change memory cell and a fabricating method thereof.
2. Related Art
A phase change memory (PCM) is a nonvolatile memory device. The resistance value of the device can be transformed by changing a crystalline phase of the phase change material through a heating effect. In other words, the PCM can be regarded as a programmable resistor which is reversibly changeable between a high resistance state and a low resistance state.
At present, a chalcogenide phase change material is widely applied in forming the memory cell of the PCM. The chalcogenide is a substance with various solid-state phases, and can be a thermo-induced transition along with a temperature variation. The chalcogenide has a high resistance value when in an amorphous state (with an irregular atomic arrangement), while has a low resistance value when in a crystalline state (with a regular atomic arrangement). Herein, the temperature variations can be achieved by providing current or optical pulses, or by other manners.
As for the structure, the coupling interface of the current path and the phase change material is designed into a small hole, to concentrate the current, such that the phase change material near the small hole has a high current density, thereby changing the phase state of the chalcogenide. The current heating effect of the resistance value on the phase change layer is a function of the area of the coupling interface. Therefore, the smaller the area of the coupling interface is, the better it is. And the higher the resistance value is, the higher the heating efficiency of unit current of the phase change material is, and therefore the operation current can be lowered.
In general, in the PCM, a transistor is used as a select device to control the current passing through the PCM cell and the voltage applied on the chalcogenide. Therefore, in order to reduce the size and power consumption of the PCM, the operation current for the PCM cell must be reduced. The current heating effect of the resistance value in the interface region is a function of the contact area of the interface region. Therefore, in the conventional technique, the object of dropping the operation current is achieved by reducing the area of the interface region between the current path and the phase change material.
Conventionally, the PCM cell is of a T-shape structure, wherein a current path through a phase change layer 130 is formed between the upper and lower electrodes 110, 120, as shown in FIG. 1. A small hole is formed on the dielectric layer 140 by a lithographic process, and then filled with a metal material to form the lower electrode 120, such that the contact area between the lower electrode 120 and the phase change layer 130 is reduced. Herein, the contact area between the phase change material (i.e. the phase change layer) and the lower electrode for heating is limited by the capability of the lithographic process. And that, the small hole is filled with a metal material, easily causes a problem of inadequate step coverage. Moreover, in actuality, it is uneasy to update the lithographic process, because the equipment must be renewed and personnel must be trained, so that a great deal of labor and costs are consumed.
Therefore, a tapered design is proposed, in which a tip of the tapered lower electrode contacts the phase change layer, thereby reducing the contact area between the two.
Referring to FIG. 2, a heating electrode 122 and multiple conductive substrates 121a, 121b, 121c, 121d, which stack in order, are etched in sync to form the tapered structure using an isotropic etching technique, and then the tapered heating electrode 122 contacts the phase change layer 130 to reduce the contact area, as shown in U.S. Pat. No. 6,800,563 B2. However, in actuality, several different materials must be taken into consideration simultaneously when the etching is carried out according to this method. Therefore, the etched pattern will be of a poor uniformity and cannot satisfy the requirements.
Furthermore, referring to FIG. 3, an etching and a photoresist lateral downsizing are carried out alternately, so as to etch the dielectric layer 142 to a tapered structure. Then a heating electrode 122 is deposited. Therefore, the heating electrode 122 at the tip contacts the phase change layer 130, thereby reducing the contact area, as shown in U.S. Pat. No. 6,746,892 B2.
Moreover, an edge contact PCM cell has been developed, as shown in FIG. 4. The heating electrode 122 is disposed in the interlayer bordering on the trench sidewall. And the contact area between the heating electrode 122 and the phase change layer 130 is controlled by the thickness of the heating electrode 122. However, there is a difficulty in filling the hole with the phase change material, and it leads to a poor contact of the lateral contact interface to cause a problem in the uniformity and reliability of the device. Moreover, the heating electrode extends transversally to contact the phase change material, such that the current path of the heating electrode is too long. Also, the heating electrode has high resistance to cause extra power consumption.
Furthermore, another PCM cell is a lateral cell, as shown in U.S. Pat. No. 6,867,425. Referring to FIG. 5, the electrodes 112, 124 are disposed in the interlayer bordering on the trench sidewall, and the contact area between the electrodes 112 and the phase change layer 130 is controlled by the thickness of the electrodes 112, 124. Herein, the operation current can be reduced by the lateral contact, and the path of the current flowing through the phase change material can be shortened by controlling the distance between the two electrodes, to reduce the power consumption when operating the device. However, the material of an heating electrode is usually of a high resistance, and when it serves as a lead, it will cause an increase in parasitic resistance, and further cause extra power consumption. Furthermore, when the distance between the two electrodes is very small, operational power consumption can be reduced, but there are the problems of difficulty, uniformity, and reliability for filling the hole with the phase change material and contact between the phase change material and the sidewall.
It can be known from the above that there are many methods for reducing the contact area between the current path and the phase change material, but in the implementation, the methods are easily limited by process equipment and/or technical ability. Therefore, many methods for reducing the contact area between the current path and the phase change material are provided, so as to be flexibly combined with the equipment and technology of the pre process and/or post process, thereby accelerating the development of this field. Thus, those skilled in the art have devoted themselves to providing a simple, highly practicable process, for reducing the contact area between the current path and the phase change material.